Address to the cathedral deputies: the architect as physician
On knowing a building's rules, the nature of weight and force, and defending his dome model
In this draft address to the 'Signori padri diputati' — the deputies of the cathedral works — Leonardo argues by analogy with medicine: just as physicians must know what man, life and health are before they can heal, so the ailing building needs a 'doctor-architect' who understands what a building is, from what rules right building derives, and what causes hold an edifice together and make it permanent. He promises to satisfy them partly by reasoning and partly by works, citing the authority of ancient architects and the causes of buildings' ruin and endurance, especially the effect of load. He offers to expound the first architect's intention for the cathedral and to show that his own model shares the symmetry and correspondence proper to the founded edifice, urging them to set aside all passion in judging it.
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The doctor-architect and the rules of right building
As physicians must grasp what man, life and health are, the sick building requires a doctor-architect who understands what a building is, from what rules right building derives, into how many parts they are divided, and what causes hold the edifice together and make it lasting. Whoever has true knowledge of these will leave the deputies satisfied by his reason and work.
The nature of weight and the desire of force
The architect must know the nature of weight and what the desire of the force is, and in what way the members should be woven and bound together, and what effect they bring forth once joined. Leonardo proposes to demonstrate effects from their causes and to confirm reasons by experience.
Defending his model and asking for impartial judgement
Promising not to detract from or defame anyone, he offers to expound the original architect's intention for the cathedral and to show that his own model has the symmetry, correspondence and conformity proper to the founded edifice. He closes: whether he or another demonstrates it better, take it — set aside every passion.